Overview Of Pneumonia Health Article

Even with intensive laboratory investigation, the specific microbiologic cause can be established with certainty only in approximately 50% of patients with pneumonia. The likely predominant organism varies based on the host's epidemiologic factors, the severity of illness, and which laboratory approach is used to establish the diagnosis. S. pneumoniae is the organism most frequently detected by culture of the sputum or blood. In contrast, Mycoplasma pneumoniae is frequently detected with serologic tests. Additional bacterial agents include Haemophilus influenzae , Staphylococcus aureus , enteric gram-negative bacilli, and L. pneumophila . Chlamydia and respiratory viruses have also been implicated in up to 10% of cases. The so-called atypical pathogens, including M. pneumoniae , Chlamydia pneumoniae , and Legionella species, are being increasingly recognized as important and prevalent causes of pneumonia. In addition, mixed infections particularly related to co-infection with these "atypical" pathogens, have been reported in up to one third of patients with lower respiratory tract infection. Specific host factors also influence the relative risk for infection with specific microorganisms. For instance, smokers and those with COPD are at increased risk for invasive S. pneumoniae , as well as H. influenzae , M. catarrhalis , and Legionella . Alcoholism is associated with increased risk for drug-resistant S. pneumoniae , anaerobic lung infection, and tuberculosis. The clinician also should solicit information about household and workplace exposures, such as to other ill persons, as well as travel to specific geographic regions, such as the central United States, where histoplasmosis is common, or the Southwest, where coccidioidomycosis is found, or environmental exposures such as to birds (psittacosis), bats (histoplasmosis), rabbits (tularemia), and farm animals (Q fever).

Although most attention focuses on bacterial causes of severe community-acquired pneumonia, viruses can also cause serious lower respiratory tract infections. Recently, severe acute respiratory syndrome (SARS), which is caused by the SARS-associated coronavirus (SARS-CoV), has been identified as the cause of rapidly progressive respiratory insufficiency with a case fatality rate of 4 to 15%, depending on the age and geographic location of the patient. SARS should be considered in any patient who, within the past ten days, has traveled (including transit in an airport) to an area with documented or suspected SARS or had close contact with a person known or suspected to have SARS. Signs and symptoms of SARS include: temperature >100.4° F (>38° C) and one or more clinical findings of respiratory illness (e.g., cough, shortness of breath, difficulty breathing, or hypoxia), particularly if there is radiographic evidence of pneumonia or clinical evidence of the respiratory distress syndrome without another identifiable cause. The Centers for Disease Control and Prevention (CDC) maintains updated lists of regions with community transmission of SARS and management of SARS through their website (www.cdc.gov).

SARS is highly contagious, and lethal transmission to health care workers has been documented. Isolation procedures and equipment appropriate for SARS include standard, contact, and airborne isolation precautions such as scrupulous hand hygiene, gowning, disposable gloves, the use of N95 respirators, and eye protection to prevent transmission in health care settings. Suspected cases of SARS require notification of local public health departments and the CDC. The CDC has access to appropriate testing for case confirmation, including detection of antibody to SARS-CoV, detection of SARS-CoV RNA by RT-PCR, and the ability to isolate SARS-CoV from specimens.

In addition to SARS, other respiratory viruses that can cause severe pneumonia include influenza and respiratory syncytial virus (RSV). Both influenza virus and RSV can be detected in respiratory secretions, which should be obtained in suspected cases. It is estimated that influenza infections are responsible for between 25,000 to 50,000 deaths annually in the United States, predominantly in elderly patients and in patients with underlying cardiopulmonary or metabolic diseases. Influenza-associated pneumonia should be considered in the differential diagnosis of respiratory infections in high-risk patients with underlying disease and in residents of nursing homes or other chronic care facilities during the season of October through May, especially in patients who have not received appropriate vaccination. It is also being increasingly appreciated that RSV, though formerly considered mainly an infection in pediatric populations, can lead to serious lower respiratory tract infections in adults during the winter season. Host immunity to RSV infection in childhood is incomplete, and recurrent infections can occur in both immune-competent and immune-impaired adults, particularly in the elderly. An effective vaccine for RSV is not currently available.

Clinical suspicion of pneumonia should prompt standard posteroanterior and lateral chest radiography. Although the pattern of infiltration can rarely establish a specific microbiologic etiology, the chest films are most useful for providing essential information on the distribution and extent of involvement, as well as potential pneumonic complications. Many bacterial pneumonias result in localized alveolar infiltrates and consolidation. Even though pneumococcal pneumonia is classically described as having a lobar distribution, the pattern can be multilobar (Fig. 92-1) or bilateral. The "bulging" fissure sign, which represents lobar filling and consolidation, has traditionally been attributed to Klebsiella pneumoniae , but this finding is not specific and can be observed with S. pneumoniae and other bacteria, and even with bronchoalveolar carcinoma. Diffuse interstitial and alveolar infiltrates should suggest viral infections (cytomegalovirus, influenza, or respiratory syncytial virus), L. pneumophila , or enteric gram-negative pneumonia, particularly in neutropenic patients. These diffuse pulmonary infiltrations can be indistinguishable from other causes of the adult respiratory distress syndrome. Diffuse alveolar and interstitial infiltration can also be observed in patients with Pneumocystis carinii pneumonia related to immune suppression, such as in AIDS. Cavitary lesions often indicate a necrotizing infection related to S. aureus, M. tuberculosis (Fig. 92-2), and certain endemic fungi, such as Coccidioides immitis, Aspergillus infection in the immunocompromised patient, or anaerobic lung infection with abscess formation.

The chest radiograph provides further important information about potential infectious complications of pneumonia. Pleural effusions, which occur in a variety of respiratory infections, are best documented with lateral decubitus views. The discovery of any pleural effusion of greater than 10-mm thickness on a lateral decubitus film or of any loculated effusion should prompt thoracentesis to aid in the identification of a complicated parapneumonic effusion or empyema, which may require definitive drainage. Enlargement of mediastinal and hilar lymph nodes, which is rare in acute bacterial infection, suggests fungal or mycobacterial infection, or an underlying lung cancer. Loss of volume of a lung segment or lobe should raise suspicion of postobstructive pneumonia distal to an endobronchial lesion caused by a neoplasm, occult foreign body, or broncholithiasis.

Considerable controversy exists over the appropriate microbiologic evaluation of patients with suspected pneumonia. Despite intensive microbiologic evaluation, a specific organism may not be discovered in half of patients with pneumonia. Furthermore, the majority of patients with pneumonia satisfactorily respond to simple, relatively nontoxic antibiotic regimens based on the most likely organisms causing infection. Thus, the necessity to document the precise cause of the process remains uncertain.

Debate continues over the need to perform a sputum examination with Gram staining in every patient with community-acquired pneumonia. An American Thoracic Society consensus panel has recommended that a sputum Gram stain and culture be obtained primarily if an organism that is resistant to the usual empirical treatment regimens is suspected. To be useful, sputum should contain fewer than 10 squamous cells and more than 25 leukocytes per low-power field; a well-performed Gram stain may reveal a single, predominant organism such as encapsulated gram-positive cocci (pneumococci) or small pleomorphic gram-negative coccobacilli ( Haemophilus ). However, current data have not clearly correlated Gram stain findings with the results of cultures of alveolar materials in large numbers of patients with community-acquired pneumonia. Nevertheless, sputum examination can strongly support the diagnosis of certain specific infections, including M. tuberculosis (acid-fast stain), endemic fungi (KOH preparations), P. carinii (methenamine silver or fluorescent antibody stain), or Legionella species (direct fluorescent antibody staining). In most cases of community-acquired pneumonia, the general intent of sputum Gram stain examination, if it is performed, should be to detect additional or unusual pathogens, and hence to expand rather than to narrow the initial antibiotic therapy. All too often, an adequate sputum specimen cannot be obtained, and the Gram stain interpretation may be equivocal. Therefore, an initial therapeutic plan must be formulated based on the most likely pathogens responsible for the pneumonia.

If unusual or drug-resistant pathogens are suspected, sputum specimens should be sent for culture before antibiotic therapy is initiated. When the culture results are available, they should be compared with the predominant organisms observed on the Gram stain. Unfortunately, the sensitivity and specificity of sputum culture are not optimal, each being roughly 50%. Antibiotic susceptibility information on an isolated pathogenic organism can, however, be useful both for epidemiologic surveillance and for management of patients who do not respond to initial empirical therapy. Cultures of normally sterile body fluids such as blood, pleural fluid, or occasionally cerebrospinal fluid (CSF) are highly specific when positive. Approximately one fourth of patients with bacterial pneumonia have demonstrable bacteremia. Blood cultures should be obtained before antibiotic administration in patients with serious illness due to pneumonia, and diagnostic thoracentesis should be performed if an effusion is large enough to be aspirated safely. CSF examination is usually reserved for patients with additional signs and symptoms of meningeal irritation or abnormalities on the neurologic examination.

Invasive sampling of respiratory secretions is usually not necessary in patients with community-acquired pneumonia. Flexible fiberoptic bronchoscopy with a protected catheter brush and bronchoalveolar lavage sampling has largely supplanted transtracheal and transthoracic needle aspiration. Bronchoscopy is indicated in selected clinical situations in which a delay in accurate diagnosis may have serious consequences, such as in immunocompromised hosts or patients whose conditions have worsened despite initial antimicrobial therapy. Other indications for bronchoscopy in the setting of apparent community-acquired pneumonia include either lung abscess detected on the chest radiograph or evidence of volume loss and distal consolidation suggesting an endobronchial obstruction.

Immunologic techniques, such as immunofluorescence, enzymelinked immunosorbent assay, antigen detection, polymerase chain reaction, and DNA hybridization, may be considered when specific organisms are strongly suspected on clinical grounds, but these tests are not routinely indicated in most cases of community-acquired pneumonia. For example, Legionella urinary antigen screening and acute and convalescent serologies may be helpful when L. pneumophila pneumonia is suspected. Furthermore, the judicious use of fungal serologies can detect endemic mycoses, particularly histoplasmosis and coccidioidomycosis. Bronchoscopy with lavage for immunostaining may, in selected circumstances, provide enhanced sensitivity, such as in the diagnosis of P. carinii pneumonia.